An exhaust gas of automobiles which use gasoline for fuel contains hazardous components such as hydrocarbon (THC), carbon monoxide (CO), and nitrogen oxide (NOx) Therefore, it is necessary to purify each of the hazardous components in such a manner that the hydrocarbon (THC) is converted into water and carbon dioxide by oxidation; the carbon monoxide (CO) is converted into the carbon dioxide by oxidation; and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst (hereinafter, referred to as an “exhaust gas purification catalyst”) adapted to treat these exhaust gases, three way catalysts (TWC) capable of being used for a redox of CO, THC, and NO have been used.
Three way catalysts are well known, in which a precious metal such as Pt, Pd, and Rh is supported on a refractory oxide porous material having a high-specific surface area, for example, an alumina porous material having a high-specific surface area and the precious metal is supported on a substrate, for example, a monolithic substrate made of a refractory ceramic or metallic honeycomb structure or on refractory particles.
Since purification characteristics (three way characteristics) for CO, KC, and NOx due to the three way catalysts are closely related to a stoichiometric air-fuel ratio (A/F) representing the atmosphere of an automobile exhaust gas, the three way catalysts were generally controlled to the following conditions in the past: A/F was around 14.6 (stoichiometric air-fuel ratio) in which exhaust gas purification function was sufficiently exhibited, in other words, an excess air ratio λ was near 1.
In recent years, from the viewpoint of improving fuel efficiency and reducing the carbon dioxide emission amount, an engine is desirably controlled in a slightly lean area in which the air-fuel ratio is higher than the stoichiometric air-fuel ratio (A/F=14.6), that is, satisfies a level of 14.6<A/F≦16.0. In the case of a slightly lean control to make a lean combustion in an oxygen-rich atmosphere, since the amount of fuel to be used is reduced, the fuel efficiency can be improved and the generation of CO2 as a combustion exhaust gas can also be suppressed.
However, since the oxygen becomes excessive and the purification performance of exhaust gas purification catalyst for NOx is extremely deteriorated under the lean condition as described above, catalysts have been desirably developed which can purify NOx with high efficiency even under the lean condition.
As the catalyst capable of purifying NOx with high efficiency even under the lean condition, the applicant has proposed a catalyst in which Rh is supported on a carrier consisting of zirconium phosphate (Patent Document 1).